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United States Department of Agriculture

Agricultural Research Service

Title: In-Stream Wetland Mitigation of Nitrogen Contamination in a Coastal Plain Stream

Authors
item Hunt, Patrick
item Stone, Kenneth
item Humenik, Frank - NC STATE UNIV
item Matheny, Terry
item Johnson, Melvin

Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 24, 1998
Publication Date: N/A

Interpretive Summary: Nonpoint source nitrogen is a major water pollution problem because nitrogen stimulates growth of aquatic flora and microbes, which can lower oxygen in the water. Low oxygen can kill or displace both fish and shell fish that are economically and environmentally important. Unfortunately, the excess nitrogen loading of streams and lakes is widespread. As part of a USDA Water Quality Demonstration Project in the Coastal Plain of North Carolina, we evaluated removal of nitrogen from a nitrogen-contaminated stream using an in-stream wetland (ISW). Wetlands are one of the most effective ecosystems for the transformation and removal of nitrogen. Our ISW was established at the exit of a 1,000-acre watershed. Even though it was less than 1% (8 acres) the size of the watershed, the ISW dramatically lowered stream nitrate-N (2.8 pounds/acre/day). The ISW functioned best during the warmer months, but it removed more than 50% of the nitrogen that annually entered the wetland. We concluded that ISW's are very good landscape features for mitigation of excess nonpoint source nitrogen and buffers of low level nitrogen loads that move from natural and agricultural ecosystems in the southeastern Coastal Plain of the USA. As such, they are a good complement to other best management practices for improved water quality.

Technical Abstract: Nonpoint source nitrogen from riverine origin is a major water quality problem throughout the world, and stream nitrogen is often acquired from agricultural watersheds. As part of a USDA Water Quality Demonstration Project in the Coastal Plain of North Carolina, we evaluated removal of nitrogen from a contaminated (6.7 mg/L, nitrate-N) stream using an in-stream wetland (ISW). Wetlands are one of the most effective ecosystem for the transformation and removal of nitrogen. Our ISW was established at the exit of a 425-ha watershed. It ranged in depth from about 0.3 to 3 m, and it was less than 1% (3.3 ha) the size of the watershed. However, the ISW dramatically lowered stream nitrate-N from 6.7 to 1.9 mg/L. The mean annual N removal rate of the wetland was approximately 3.1 kg/ha/day, and more than 50% of N entering the wetland was removed. In the warmer months when denitrification was enhanced, nitrate-N mass removal was highly correlated to nitrate-N load (r2 = 0.93). Ammonia-N mass removal was highly correlated to ammonia-N load (r2 =0.85) during the cooler months but not during the warmer months. Thus, ISW's appear to be very good landscape features for mitigating excess nonpoint source nitrogen that moves from natural and agricultural ecosystems in the southeastern Coastal Plain of the USA. As such, they are a good complement to other best management practices for improved water quality.

Last Modified: 8/22/2014
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